1. Field of the Invention
The invention relates to the purification of alkylene carbonates, also known as glycol carbonates, and more particularly to the removal of organic halide compounds from alkylene carbonates.
2. Description of Related Methods
The reaction, of alkylene oxides with carbon dioxide in the presence of a catalyst is the conventional route to the preparation of alkylene carbonates. U.S. Pat. No. 2,773,070 to Lichtenwalter et al. discloses a process for preparing alkylene carbonates using an ammonium halide catalyst. U.S. Pat. No. 2,873,282 to Mc Clellan discloses the use of certain quaternary ammonium compounds to catalyze the reaction of alkylene oxides and carbon dioxide. W. J. Peppel, in "Preparation and Properties of the Alkylene Carbonates," Industrial and Engineering Chemistry, vol. 50, no. 5, pp. 767-770 (May 1958), provides an overview of the various methods then known for the preparation of alkylene carbonates.
It appears that most of the known processes employ halogen-based catalysts. For example, U.S. Pat. No. 4,786,741 to Sachs teaches a process for preparing alkylene carbonates that employs a catalyst selected from the group consisting of organic quaternary ammonium halides, organic quaternary phosphonium halides, organic sulfonium halides, and organic antimony halides. European Patent Application 0 297 647 claims a process wherein alkylene carbonates are prepared using a catalyst comprising an alkali or alkaline earth metal halide. Japanese Patent Application Number 63-181765 also discloses a method for the preparation of alkylene carbonates using an alkali halide catalyst. Typical halogen-containing catalysts are Friedel-Crafts catalysts, such as AlCl.sub.3, FeCl.sub.3, SnCl.sub.4, BF.sub.3 and other Lewis acids, and Ziegler catalysts, such as combinations containing a transition-metal halogen compound and a metal hydride or metal alkyl.
Halide-based catalysts, however, tend to contaminate the alkylene carbonate product with halogen compounds. These halogen contaminants are present both as ionic halides (residual catalyst) and as organic halide compounds. In the case of volatile alkylene carbonates, careful distillation removes the carbonates overhead while leaving ionic halides in the distillation bottoms. In processes where the product may be contaminated with ionic halides, it is known to remove ionic halides in the form of residual catalyst by contacting the contaminated product with one of a variety of adsorbents. For example, U.S. Pat. No. 4,547,620 to Miyata et al. discloses the use of a hydrotalcite to remove ionic halides in the form of residual catalyst from contaminated products.
Organic halides, however, generally are much more difficult to remove. In an organic halide, the halide is covalently bonded rather than being the free ionic species that it is in an ionic halide. Additionally, organic halide contaminants, such as BrCH.sub.2 CH.sub.2 OH, BrCH.sub.2 CH.sub.2 OCH.sub.2 CH.sub.2 OH, BrCH.sub.2 CH.sub.2 OCH.sub.2 CH.sub.3, BrCH.sub.2 CH.sub.2 OCH.sub.2 CH.sub.2 OCH.sub.2 CH.sub.3, and BrCH.sub.2 CH.sub.2 OCH.sub.2 CH.sub.2 Br, that are formed, for example, when ethylene carbonate is prepared using a tetraalkyl ammonium bromide catalyst, have a boiling point close enough to the boiling point of ethylene carbonate to make separation by distillation very difficult. Applicants, however, have discovered that alkylene carbonates that are contaminated with organic halide compounds may be purified by contacting the contaminated alkylene carbonates with hydrotalcite. Until the discovery disclosed in the present application, it was unknown that organic halide compounds could be removed from contaminated products using hydrotalcite.